JP2004508598A - Audio signal compression - Google Patents

Abstract

A method of estimating the compression gain obtained when compressing a given audio signal is to extract the signal power in a selected frequency band of the given audio signal and to calculate the compression gain by correlation with the extracted signal power. Get an estimate.

Description

[0001]
The present invention relates to audio signal compression.
[0002]
International Patent Application WO 98/16014 discloses a data compression device for compressing an audio signal. The data compression apparatus includes an input terminal for receiving an audio signal, a 1-bit A / D converter for A / D converting the audio signal to obtain a bit stream, and performing a lossless data compression step on the bit stream signal. It includes a lossless coder that obtains a compressed bit stream signal, and an output terminal that supplies a data compressed bit stream signal. Further, a recording device and a transmitter device including a data compression device are disclosed. Furthermore, a data decompression device for decompressing a data-compressed bit stream signal supplied by the data compression device, and a reproducing device and a receiver device having the data decompression device are also disclosed.
[0003]
It is an object of the present invention to provide an advantageous compression gain estimation during audio compression. To this end, the present invention provides a method and apparatus for estimating compression gain, a recording device, and a transmitter as described in the independent claims. Advantageous embodiments are described in the dependent claims.
[0004]
The compression gain estimation can be used, for example, for authoring and / or editing an audio signal sequence, such as a piece of music. Compression gain estimation is used to control signal processing parameters, such as adaptive control of the compression ratio by compression encoding, and to provide a quick estimate of the amount of data that can be stored or recorded on a storage or recording medium, thereby providing a super audio compact For example, one piece of music content can be recorded in a finite maximum storage content of a recording medium such as a disc (SACD).
[0005]
Theoretically, to obtain such an estimate, use the average actual compression ratio or coding gain for a portion of the audio signal, such as a piece of music, as an indication of the compression ratio for the entire audio signal sequence. It is possible. However, in such an approach, a typical audio signal sequence in the form of a piece of music has a fairly wide compression ratio or coding gain variation with a fairly short time correlation. Thus, to obtain a compression ratio estimate with the desired level of precision or accuracy, a very large portion of the signal sequence must be used. Due to the computation time requirements, such a solution would not be really acceptable.
[0006]
According to a first aspect of the invention, an advantageous estimate of the resulting compression gain is to extract the signal power contained in the selected frequency band of the audio signal and to make that estimate by correlation with the extracted signal power. obtain. A first aspect of the invention is based on the insight that a quick estimation of the compression ratio or coding gain is obtainable by using a correlation between the signal power of the audio signal and the compression gain. This aspect is particularly advantageous, for example, when losslessly compressing a 1-bit bit stream signal in a direct stream digital format.
[0007]
It is well known that the coding gain obtained from standard PCM coding is directly related to the degree of structure (flatness) in signal power. However, in the above aspects of the invention, the signal power itself is used, and no structure is used.
[0008]
Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.
[0009]
Looking at lossless encoders, for example, in the audio signal band itself up to 20 kHz, a relatively flat response curve to compression ratio exists as a function of signal power, and the compression gain cg, which depends on signal power sp, is quite large. It is shown that a steep response is obtained for signal bands outside the audio signal band. This is shown, for example, in FIG. 1 for a signal band of 20 to 50 kHz.
[0010]
The signal processing device shown in the block diagram of FIG. 2 uses this outstanding correlation to obtain an estimate of the compression ratio or coding gain obtained by lossless coding. An input audio signal, which may be analog or digital, is provided to a signal processor 1 where the input audio signal is processed depending on at least one variable processing parameter determined by a parameter controller 2. In the illustrated example, the control signal generated by the control device 2 controls the adaptation of the adaptive sigma-delta modulator 3, and the modulator 3 outputs so-called DSD (direct stream digital) data such as a 1-bit bit stream signal. Form a stream signal, which is supplied to the lossless encoder 4 as an output signal from the signal processor 1. The compression gain obtained in the encoder 4 is significantly affected by the variation of the variable parameters used to control the signal processor 1, which results in a recording such as a SACD recording on a DVD disk 5 shown in FIG. It can strongly affect the amount of data that can be recorded or stored by the medium. See Applicant's co-pending application having the same priority date as the present application under the name "Audio signal processing".
[0011]
According to an embodiment of the present invention, the correlation between the signal power of the bit stream signal in DSD format and the compression gain as shown in FIG. 1 is used to provide a quick and accurate estimation of the coding gain. As shown, the signal power is extracted from the bit stream signal by an extractor and correlator 6 connected to the output from the signal processor 1 and the compression ratio or coding gain estimation is performed as a parameter control 2 Supplied to
[0012]
As shown, the component of the parameter controlled signal processor 1 is typically an adaptive sigma delta modulator 3. Several approaches can be used to adapt or change the structure of the sigma-delta modulator. For example, there are approaches to shifting between structures of sigma-delta modulators having different orders, or forming structures in the high frequency noise of the modulator.
[0013]
Thus, by shifting between third-order, fifth-order, or seventh-order modulator structures, the compression ratio will generally be about 3.7 relative to the third-order structure. From the range of only 2.3 or less for the structure of the seventh order, which means that the amount of data of the compressed bit stream signal from the encoder 4 can be reduced by a recording medium such as a DVD disk 5. Used to fit the limited recording or storage capacity of
[0014]
FIG. 3 shows a preferred topology of a fifth order sigma-delta modulator for use in the signal processor 1 shown in FIG. The illustrated topology is based on a number of resonator structures, where the coefficients c1, c2, ..., c5 in the feedback loop of the resonators R1, R2, ..., R5 are the poles of the loop filter (or the zero of the noise transfer function). ). Although the topology shown is for a fifth order modulator, the same topology may be used for a seventh order modulator simply by adding another resonator structure.
[0015]
In a typical design of a sigma-delta modulator, the poles are usually located in the audio band. However, it is preferred in the present invention that the at least one pole be located outside the audio band, forming an additional structure in the high frequency portion of the sigma delta spectrum (otherwise it is generally flat).
[0016]
Thus, in the standard design of a sigma-delta modulator, the poles are typically located at 8.7, 15.7, and 19.5 IkHz, but in the present invention, the last pole is located at a higher frequency from the 20 kHz region. It is preferred to shift to a frequency in the region, for example 300 kHz or higher, which leads to a considerable increase in the compression gain. This can slightly degrade the signal-to-noise performance, but it is usually quite acceptable because extra noise is present at higher frequencies that are less audible to the human ear. This is because it occurs in the band.
[0017]
Shifting the pole position from the 20 kHz region to higher frequencies is achieved by adding a separate additional bandpass filter to the current modulator structure, for example, in parallel with a low pass loop filter. By using a second-order Butterworth bandpass filter as a parallel filter as described above, a significant increase in compression gain is achieved, the modulator is still stable for large inputs, and the signal to noise in the audio band is reduced. The performance remains substantially unchanged compared to the modulator which has not been modified.
[0018]
Shifting between different sigma-delta modulator structures may cause a variation in the relationship between signal power in some frequency bands of the bitstream signal and the compression ratio or coding gain, and thus the compression ratio It is advantageous to first perform a calibration process to fix the correlation by taking into account the fact that it may affect the correlation used as a basis for the estimation.
[0019]
For this purpose, a fixed sigma delta modulator is incorporated as a final processing device before the lossless encoder 4. This is indicated by the dashed block 7 in FIG. Therefore, the signal power is preferably extracted from the signal supplied from the fixed sigma delta modulator 7 to the lossless encoder 4.
[0020]
In the above-described signal processor embodiment shown in FIG. 2, an adaptive sigma delta modulator is used to adapt or change the compression ratio or coding gain in response to a correlation derived from the signal power of the bitstream signal. Other approaches, such as reducing the signal level of the built-in bandwidth-limited low-pass filter in the signal processor, before the fixed or adaptive sigma-delta modulator, may also be used separately or in combination.
[0021]
Furthermore, the signal processing device performing the method according to an embodiment of the present invention may provide an estimate of the compression ratio obtained in the subsequent compression coding, for example, for a pure evaluation, formatting, authoring and / or editing of the audio signal sequence. It need not itself include compression encoding means as long as it can be used separately in the studio environment for.
[0022]
It should be understood that the above-described embodiments are illustrative of the present invention and not limiting. One skilled in the art can design many alternative embodiments without departing from the scope of the claims. In the claims, any reference signs placed between parentheses shall not limit the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct components, and by means of a suitably programmed computer. In the device claim, some means are enumerated, but some of the means may be embodied as identical hardware items. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
[Brief description of the drawings]
FIG.
FIG. 4 is a diagram illustrating a relationship between a compression gain and a signal power in a selected frequency band of an audio signal.
FIG. 2
FIG. 2 is a simplified block diagram showing a digital audio signal recording or transmission chain into which an embodiment of the signal processing device of the present invention is incorporated.
FIG. 3
FIG. 3 is a simplified topology diagram illustrating a fifth order sigma delta modulator used in the signal processor shown in FIG. 2.

Claims (11)

A method for estimating the compression gain obtained when compressing a given audio signal, comprising:
Extracting signal power in a selected frequency band of the given audio signal;
Obtaining an estimate of the compression gain by correlation with the extracted signal power. The method of claim 1, wherein the audio signal is a one-bit noise-shaped digital signal. The method according to claim 1, wherein the compression is a lossless method. The method of claim 1, wherein the selected frequency band is equal to or greater than 20 kHz. The method of claim 4, wherein the selected frequency band is between 20 and 50 kHz. A method for recording an audio signal on a recording medium, comprising:
Compressing the audio signal to obtain a compressed audio signal;
Recording the compressed audio signal on the recording medium.
The method further comprises:
Estimating the compression gain obtained in the compression step by extracting signal power in a selected frequency band of the audio signal and obtaining an estimate of compression gain by correlation with the extracted signal power;
Controlling the compression step depending on the estimated compression gain. 7. The method of claim 6, wherein said compressing comprises converting said audio signal to a digital signal by adaptive noise shaping modulation in response to said estimated compression gain. A method of transmitting an audio signal,
Compressing the audio signal to obtain a compressed audio signal;
Transmitting the compressed audio signal via a transmission medium,
The method further comprises:
Estimating the compression gain obtained in the compression step by extracting signal power in a selected frequency band of the audio signal and obtaining an estimate of compression gain by correlation with the extracted signal power;
Controlling the compression step depending on the estimated compression gain. An apparatus for estimating a compression gain obtained when compressing a given audio signal,
Means for extracting signal power in a selected frequency band of the given audio signal;
Means for obtaining an estimate of said compression gain by correlation with said extracted signal power. A recording device that records an audio signal on a recording medium,
Means for compressing the audio signal to obtain a compressed audio signal;
Means for recording the compressed audio signal on the recording medium,
The recording device further comprises:
An apparatus according to claim 9 for estimating a compression gain obtained during the compression,
Means for controlling said compression in dependence on said estimated compression gain. A transmitter for transmitting an audio signal,
Means for compressing the audio signal to obtain a compressed audio signal;
Means for transmitting the compressed audio signal via a transmission medium,
The transmitter further comprises:
An apparatus according to claim 9 for estimating a compression gain obtained during the compression,
Means for controlling said compression in dependence on said estimated compression gain.